feat(auth): PKCE on the device login flow

[Zingzy]

Adds PKCE to spoo login, which the backend now requires on every device authorization.

What changed

• Generate a fresh S256 verifier/challenge pair per login (internal/auth/pkce.go). The verifier is a 43-char base64url string (32 random bytes), the minimum RFC 7636 length.
• GET /auth/device/login now carries code_challenge and code_challenge_method=S256 alongside the existing app_id, redirect_uri, and state.
• DeviceFlow.Run returns the one-time code together with the verifier; ExchangeDeviceCode sends code_verifier in the POST /auth/device/token body.

Compatibility

Older CLI builds fail at the new login step with the backend's "update the app" error page. Nothing else in the flow changes: the loopback callback, state check, and stored token shape are untouched, and existing sessions keep working.

Testing

go build ./..., go vet ./..., and go test ./... all pass. New tests cover the PKCE derivation (including the RFC 7636 Appendix B fixed vector), that the login URL carries a 43-char S256 challenge, that the returned verifier hashes to it, and that the exchange body includes code_verifier.

Pairs with the backend change (spoo-me/spoo#260).

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 815f5458-7d4f-4948-9434-fee8b2690b93

📥 Commits

Reviewing files that changed from the base of the PR and between d436ffe and 37dcdbd.

📒 Files selected for processing (7)

• internal/api/auth.go
• internal/api/auth_test.go
• internal/auth/device.go
• internal/auth/device_test.go
• internal/auth/pkce.go
• internal/auth/pkce_test.go
• internal/cmd/auth.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/device-auth-pkce

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[Zingzy]

Closing as a duplicate. This was opened by a background agent that did not see the already-open PR. The canonical PR with the same change (and more) is #1. Reopen if needed.